In a thermal treatment of a substrate, such as a semiconductor substrate, it is required to have a noncontact method for measuring the substrate temperature in order to perform a highly-accurate, highly-efficient thermal treatment. A radiation thermometer has been commonly used as a noncontact temperature measuring apparatus. However, it is difficult to accurately measure the temperature of a substrate with the radiation thermometer due to the emissivity varying according to the condition of the substrate surface. Thus, a method for measuring the temperature of a substrate using a laser has been proposed.
For example, Patent Document 1 proposes a method for determining the temperature of an object of temperature measurement. The method includes the steps of: irradiating both the object of temperature measurement and a reference member with a probe laser beam emitted from a semiconductor laser; detecting a synthesized light of a reflected light from the object of temperature measurement and a reflected light from the reference member; acquiring, from the spectrum of the synthesized light, the frequency of a light component which specifically exhibits a variation in intensity in response to the temperature of the object of temperature measurement; and thereby determining the temperature of the object of temperature measurement on the basis of the frequency.
Patent Document 2 proposes a method for temperature measurement by using a surface temperature measuring apparatus. The surface temperature measuring apparatus for measuring the surface temperature of a work object includes the following units: a probe laser beam irradiation unit for irradiating a measurement point on the work object with a probe laser beam; a probe laser beam separation unit for separating the probe laser beam radiated from the probe laser beam irradiation unit, and then radiating the separated probe laser beams in parallel on the measurement point on the work object and a reference point located a predetermined distance from the measurement point; a pulse laser beam irradiation unit for heating the measurement point intermittently by radiating a pulse laser beam on the measurement point; an interferometer for detecting interference by collecting reflected lights reflected from the measurement point and the reference point; and a calculation unit for calculating the temperature of the measurement point on the basis of the frequency of an ultrasonic oscillation at the measurement point which has been obtained by the interferometer.
Patent Document 1: Japanese Patent Application Laid-open Publication No. 2000-162048
Patent Document 2: Japanese Patent Application Laid-open Publication No. Hei 11-190670